Also just adding to this thread as I want this to be an option as well. I know it existing on all the other major players in this space, by why not Quickbooks? I have clients ask how to leave a tip fairly regularly and I am actually a little embarrassed when I have to say it is not a functionality of the software. 

The only workaround I have found is to accept E-Transfers, match them to an Invoice and then reconcile the difference. However, most of my clients do not want to pay by this means. 

I am not sure why it is taking Quickbooks so long to implement such a simple box on an invoice, especially when it is clear that it is a requested feature. Does Quickbooks not recognize that it would result in additional money for the company? Larger total invoices via a tip, would more a larger processing fee % for Quickbooks.